This invention is related to producing oxygen gas suitable for human breathing, and more specifically to pressure swing adsorption air separation using selective molecular sieves.
It is known that some zeolitic materials have the characteristic of adsorbing nitrogen and oxygen from the air and that the selectivity of such zeolitic materials for nitrogen adsorption in relation to oxygen adsorption increases as pressure increases. Therefore, as air is injected under pressure into a zeolitic material, the zeolitic material adsorbs a greater proportion of nitrogen than oxygen. When the zeolitic material is packed in an elongated container, a stream of air injected under pressure into one end of the container is progressively stripped of a portion of its nitrogen content, which results in a proportionately higher oxygen content in the air stream toward the downstream end of the container. Therefore, the air in the downstream end of the container after it has passed through the zeolitic material under pressure is oxygen enriched.
As that flow of air through the zeolitic molecular sieve continues, the zone of increased nitrogen adsorption advances longitudinally through the length of the container toward the discharge end. When this zone or nitrogen front nears the discharge end, the effectiveness of the oxygen concentration near the discharge end diminishes. If the flow of air is continued long enough, the nitrogen front will replace the zone of oxygen enriched air at the discharge end of the container, and the effectiveness of the molecular sieve for producing oxygen will be virtually eliminated. However, by bleeding the pressure from the inlet or injection end of the container and allowing air to flow therethrough in the reverse direction, the zeolitic molecular sieve can be purged of the adsorbed nitrogen. The pressure drop resulting from bleeding and exhausting the inlet end causes the nitrogen adsorption selectivity of the zeolitic material to decrease and swing toward increased selectivity to oxygen adsorption. Consequently, the combination of bleeding pressure and reversing the flow of the air stream through the zeolitic material results in purging and exhausting the nitrogen from the container.
This selective nitrogen adsorption characteristic of zeolitic materials is used advantageously to produce oxygen for a variety of uses, including for breathing by persons, such as those with respiratory ailments, who benefit from breathing oxygen in higher concentration or purity than that naturally occurring in the air. However, in order to reach a useful oxygen purity level for breathing, it is necessary to increase the oxygen enrichment of the gas to a significantly greater extent than that achieved by one pressure and flow cycle.
The U.S. Pat. No. 4,194,891, issued to Earls et al, discloses a method and apparatus for "bootstrapping" the oxygen enrichment level of the gas through a plurality of cycles of pressurized air injection into the zeolitic material followed by bleeding off the pressure and reverse flow of air or gas through the zeolitic material. This "bootstrapping" effect is achieved by introducing a portion oxygen enriched air previously produced into the discharge end of the zeolitic material container during the bleeding and reverse flow phase of the cycle. Therefore, when the new cycle begins by injecting air under pressure into the inlet end of the container, there is a head start of increased oxygen concentration in the container. Consequently, the enriched oxygen concentration produced near the discharge end during the next phase of pressurizing and flowing air through the zeolitic material is enriched an increment greater than the preceeding cycle. This incremental increase in oxygen purity results from the oxygen added during the preceeding bleeding and purging phase.
The Earls et al Pat. No. 4,194,891 teaches the use of a plurality of containers of zeolitic molecular sieve material. The cycles in each container are not in phase with the other containers so that some oxygen enriched gas is always available from one container in the pressure and air injection phase to another container in the bleed and reverse flow path. The Earls et al patent teaches the addition of a portion of the oxygen enriched gas from one container to another container while the remaining portion is used for breathing or other useful purposes. With the repetition of these cycles over a period of time, including injection of oxygen enriched gas during bleeding and purging, each cycle achieves a higher oxygen purity level to a range of 88 to 98 percent oxygen.
While this "bootstrapping" process of repeated cycles and adding increasingly oxygen enriched gas to the purge phase of the cycles is effective to reach the level of an enriched oxygen gas suitable for breathing, some problems remain. For example, prior art methods and apparatus, such as that described in the Earls et at patent, have exceedingly complex flow circuits with numerous valves and timer control devices that must be set relative to such parameters as length of container, air injection rate, pressures and the like. They are also bulky and heavy and are quite efficient in energy consumption.
It has also been discovered that all of the prior art oxygen concentrators used for producing oxygen suitable for breathing by persons having respiratory problems are inefficient, excessive in size and weight, and excessively noisy in operation. These problems in prior art oxygen concentrators result from a misdirected, pressure oriented design concept that is common to the prior art oxygen concentrators. This misdirected design concept has resulted in part from a natural tendency to follow conventional adiabatic air compression systems as opposed to more unconventional isothermal gas flow concepts and, in part, to general unavailability of pneumatic system components conducive to isothermal air flow design concepts. The adiabatic compression systems of prior art oxygen concentrators suffer the disadvantages of excess heat inherently produced in adiabatic compression systems, excessive power requirements resulting in inefficient operation, and restricted air flow capacity resulting in lower oxygen product concentration per unit volume.
The inventors of this invention have found that the effectiveness of nitrogen adsorption in the zeolite bed decreases with increasing temperature of the air flowing into the zeolite bed. This decrease in effectiveness is a result of the zeolite's strong tendency to adsorb moisture, which reduces its ability to adsorb nitrogen. Hot air, of course, has a higher capacity to carry moisture than cold air. Therefore, hot air will not give up its moisture content to zeolite adsoprtion as easily as cold air. Consequently, hot air tends to carry the moisture farther into the zeolite bed than cold air, thus eventually decreasing the nitrogen adsorption capacity of the zeolite substantially throughout the entire zeolite bed. In contrast, if the in-take air is maintained at a lower temperature so that it has less capacity to carry moisture, the moisture in the in-take air will be stripped from the air almost immediately by the zeolite at the in-take end, thus leaving the zeolite in the remainder of the bed in a dry condition in which it retains a higher capacity to adsorb nitrogen. Further, when the moisture is concentrated in the zeolite close to the in-take end, it is more effectively and efficiently exhausted from the zeolite bed during the back flow or purging. Therefore, when the temperature of the air is kept low, less increase in pressure is required to achieve the desired oxygen production results.
It has also been found that by increasing the flow rate in isothermal conditions, the pressure gradient (i.e., change in pressure per unit of time) in the pressure swing cycle can be lowered and still attain the same oxygen production effectiveness of the zeolite bed, as well as increasing oxygen concentration purity in the resulting product. At the same time the purge phase of the cyle can occur at a faster negative pressure gradient, which increases the effectiveness of the oxygen concentration in the zeolite beds and increases product purity. Of course, increased volume of air flow through the sieve bed also results in an increased amount of oxygen product produced per unit of time. Therefore, achieving a higher pressure gradient, as opposed to high pressure, allows production of a desired oxygen purity concentration in the product through a lower total change in pressure or pressure swing, thus utilizing less power and resulting in greater effectiveness and product purity.
Consequently, the inventors have discovered that by redirecting the oxygen concentrator pneumatic air flow system to an isothermal, high volume, low pressure air flow system, as opposed to the adiabatic, high pressure and low volume air flow systems of the prior art oxygen concentrators, the combined benefits of lower temperature, decreased pressure gradient, and decreased power requirements result in a significantly increased efficiency and increased oxygen concentration capacity and purity in the end-usable product can be achieved than the prior art oxygen concentrators. They also found, however, that suitable system components necessary to achieve an isothermal, high volume and low pressure pneumatic air flow system were unavailable. They also found that conventional oxygen concentrator systems for use by persons with respiratory problems were unsuitably noisy and bulky so that they were somewhat unpleasant and annoying during use.